The invention relates to the field of optical waveguides, and the method of manufacturing waveguides.
Roughness scattering is one of the major sources of transmission loss in planar waveguides. The roughness at the core/cladding interface, arising from the waveguide patterning process, is responsible for such a scattering. Several methods are possible to reduce the scattering losses in planar waveguides. Reduction of scattering loss by annealing the waveguide at high temperature, after the waveguide patterning process, has been previously reported by Kashimura et al. in Japanese Journal of Applied Physics, Vol. 39, Jun.2000. This publication reports the loss reduction technique for a waveguide with a low index difference waveguide between the core and the cladding. GeO2-doped silica (silicon dioxide) waveguides, whose refractive index difference between the core and the cladding is ˜0.02, were used in that study.
The roughness scattering is particularly severe for high index difference waveguides where the effective refractive index difference between the core and the cladding is above 0.1. The effective refractive index difference higher than 0.1 corresponds to the waveguide single-mode cutoff dimension less than roughly 2.5 times the wavelength in the core. Yet there has been no prior art on reducing the scattering losses by subjecting high index difference waveguides to a smoothing process after the waveguide patterning.
A strip Si/SiO2 waveguide based on SOI is an example of a high index difference waveguide. A strip waveguide has a core surrounded by a cladding comprising one or more materials having different refractive indices than the core. For SOI waveguides, oxidation at an elevated temperature is one method that smoothens rough interface and thus reduces the scattering loss. Smoothing of rough surfaces of silicon after the patterning process by oxidation, followed by oxide removal, has been reported in the literature. Juan et al., Journal of Vacuum Science Technology B, Vol. 14, No. 6, Nov./Dec. 1996, report oxidation smoothing of silicon sidewalls for mirror applications while Yahata et al., Japanese Journal of Applied Physics, Vol. 37, Jul. 1998, report smoothing for MOS applications. Yet, there have been no publications on oxidation smoothing of the silicon waveguide core to reduce scattering losses in strip waveguides.
U.S. Pat. No. 5,360,982, issued to Venhuizen describes a new waveguide fabrication technology that produces smooth silicon waveguide surface. Waveguides with smooth interfaces are formed by local oxidation of the silicon substrate. This process is different from our present invention in that the waveguide is formed by oxidation in the patent, while in the invention, the oxidation step is incorporated after the waveguides are already formed by patterning.